The present invention relates to an optical waveguide and a 1.5 .mu.m-band optical amplifier in which the optical waveguide is used.
Hitherto, the light amplifying silica glass fiber containing an activation ion of Er has been used in the optical amplifier for amplifying 1.5 .mu.m optical signals. Thus, this glass fiber has played an important role in making the optical communication high-speed. In such optical amplifier, the Er ions can be excited by a pump light having a wavelength of 1.48 .mu.m or 0.98 .mu.m. As a result, the light emission of 1.5 .mu.m-band can be obtained through the stimulated emission transition of .sup.4 I.sub.13/2.fwdarw..sup.4 I.sub.15/2, as shown in FIG. 1. It is known that an optical amplifier using the excitation at 0.98 .mu.m has a lower noise than that of another optical amplifier using the excitation at 1.48 .mu.m. In recent years, the wavelength multiplex communication system has drawn much attention in the course of the increase of the communication capacity. Thus, it has been required to get a light amplifying medium which can achieve an effective light amplification in a wide band. In view of this, there have been recent studies on the widening of the band of Er-doped silica fibers and on Er-doped halide glasses and Er-doped chalcogenide glasses. It is known that an optical waveguide and an optical fiber used therefor, which are made of Er-doped halide glass, Er-doped chalcogenide glass or oxyhalide glass glasses are expected to become a light amplifying medium used for the 1.5 .mu.m -band wavelength multiplex communication. However, if the Er ions contained in these glasses are excited at 0.98 .mu.m in an optical amplifier, the excited state absorption (ESA) of the Er ions tends to occur, as shown in FIG. 1. The reason of this is that the phonon energy of halide glass, chalcogenide glass or oxyhalide glass is lower than that of oxide glass. With this, the chance of the occurrence of the multiphonon relaxation of the Er ions by the transition of .sup.4 I.sub.11/12.fwdarw..sup.4 I.sub.13/2 becomes small, and thus the fluorescent life time becomes long in the .sup.4 i.sub.11/12 level. As a result, the ESA loss of the Er ions increases, and thus the amount of the pump energy directed to the stimulated emission at 1.5 .mu.m-band is decreased. With this, the amplification efficiency is lowered. Thus, it has been impossible to prepare an optical amplifier having low noise and wide band by using halide glass, chalcogenide glass or oxyhalide glass. In order to prepare such optical amplifier, it becomes necessary to shorten the fluorescent life time of the Er ions in the .sup.4 I.sub.11/12 level, In view of this, Japanese Patent Unexamined Publication No. 8-222784 discloses a light amplifying fiber containing in its core both of a first dopant of Er and a second dopant which is at least one selected from Tb, Dy and Eu. However, this second dopant may absorb the pump light and/or the signal light and thus decrease the amplification efficiency, in case that the absorption wavelength is varied by the effect of the host material composition, or that the second dopant is added in large amount.